[time-nuts] GPDSO is working
Bob Camp
lists at rtty.us
Sat Jul 13 16:05:42 UTC 2013
Hi
Hooking the ~ +/- 10V output of a MAX 232 to one of it's CMOS logic inputs probably isn't a real good idea. Two resistors and a cheap transistor make a fine inverter in this case.
Bob
On Jul 13, 2013, at 11:29 AM, EWKehren at aol.com wrote:
> Most MAX have two sets and one could use one as an inverter if one does not
> want to add an extra IC.
> Bert Kehren
>
>
> In a message dated 7/13/2013 10:09:12 A.M. Eastern Daylight Time,
> alsopb at nc.rr.com writes:
>
> Guys,
>
> The PIC in question was knowingly programmed "upside down" with the N
> option so it could talk directly to the computer without an RS232
> converter. (input side suitably protected from -voltage levels)
>
> This works of most PC's which in actuality use 3.3 Volt logic in their
> RS232 port and input clamp highs/lows to be within the logic family
> limits.
>
> There are two serial port choices for a PIC in the PICAXE/BS2 compilers
> N and T.
>
> From the PICAXE manual.
>
> "N idles low and T idles high. When using a simple resistor interface
> use N (inverted) When using a MAX232 type interface use T"
>
> The bottom line is depending upon what your device is putting out and
> what you are talking to you may or may not need an inverter for use with
> the MAX232.
>
> Regards,
> Brian
>
> On 7/13/2013 03:10, Chris Albertson wrote:
>> You have it 100% correct. The UT+ uses "positive" logic are the logic 1
> is
>> 5-volts but the RS-232 standard uses "negative" logic. I think the
> MAX232
>> does the conversion correctly EXCEPT if you read the RS-232 standards
> they
>> use positive logic for the control signals.
>>
>>
>> On Fri, Jul 12, 2013 at 7:34 PM, Bob Stewart <bob at evoria.net> wrote:
>>
>>> Hi Brian,
>>>
>>> That's just strange. There are a whole lot of these MAX232 and MAX3232
>>> devices being sold. Hmm, I'm looking at the UT+ User's Guide, and it
> lists
>>> the voltage levels as follows. These would imply that an inverter is
>>> necessary, right? Could it be that someone programmed your PIC upside
> down
>>> - i.e. using negative logic?
>>>
>>> TTL
>>> 0 V to 0.8 V = logic 0
>>> 2.4 V to 5.0 V = logic 1
>>> RS-232 (reordered from manual to put logic 0 on top)
>>> 5 V to 15 V = logic 0
>>> -5 V to -15 V = logic 1
>>>
>>> Bob - AE6RV
>>>
>>>
>>>
>>>
>>>> ________________________________
>>>> From: Brian Alsop <alsopb at nc.rr.com>
>>>> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and
>>> frequency measurement <time-nuts at febo.com>
>>>> Sent: Friday, July 12, 2013 9:09 PM
>>>> Subject: Re: [time-nuts] GPDSO is working
>>>>
>>>>
>>>> Hi Bob,
>>>>
>>>> Here is my experience. I had a PIC that output RS232 at 0-5 volt
>>>> levels. It actually worked with my computer directly. When I added a
>>>> MAX 232 to make the levels something like -10/+10 volts. It didn't
>>>> work. That's because the MAX232 inverts the polarity. Look at the
> data
>>>> sheet, the level converters are clearly inverters.
>>>>
>>>> The fix in my case was to invert the RS232 stream output by the PIC and
>>>> all was fine.
>>>>
>>>> I'm not sure exactly what you have but a scope sorts it out quickly.
>>>>
>>>> 73 de Brian/K3KO
>>>>
>>>>
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>>
>>
>>
>
>
>
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